Cryostats



F. J. WEB

CRYOSTATS Filed Sept. 2, 1964 2 Sheets-Sheet 1 F. J. WEBB Sept. 13, 1966 CRYOSTATS 2 Sheets-Sheet 2 Filed Sept. 2, 1964 INVENTOR. FREDERICK d, WEBB United States Patent 3,271,966 CRYOSTATS Frederick John Webb, Chilton, Berkshire, England, as-

signor, by mesne assignments, to the United States of America as represented by the United States Atomic Energy Commission Filed Sept. 2, 1964, Ser. No. 394,092 Claims priority, application Great Britain, Feb. 24, 1964, 7,655/ 64 Claims. (CI. 62-45) This invention relates to cryostats. The invention described herein was made in the course of, or under a contract with the United States Atomic Energy Commission.

According to the present invention, a cryostat comprises a vessel adapted to contain a refrigerant having a hemispherical roof surface boiling point below the ambient temperature, the lower part of the vessel forming a reservoir and the upper part of the vessel including a region in which a body to be maintained below the ambient temperature is arranged to be situated, a dome within the reservoir beneath which gas evolved by evaporation of the refrigerant in the reservoir collects, and means to vent gas from beneath the dome, the arrangement being such that evaporation of the refrigerant tends to raise the level of the top surface of the refrigerant and this level can be restored by venting the gas from beneath the dome.

Preferably a mechanism is provided to control the venting of gas from beneath the dome so that a desired level of the top surface of the refrigerant is substantially maintained. This mechanism may include a vent pipe leading from a point beneath the dome, a valve in the pipe, and a float which controls the valve in dependence upon the level of the top surface of the refrigerant.

The refrigerant may be liquid argon.

The invention enables a cryostat to be made in which the level of the top surface of the refrigerant tends to rise as the refrigerant evaporates, hence a large reservoir can be provided at a level below that of the body to be cooled and it is only necessary to refill the cryostat with refrigerant at fairly long intervals.

A cryostat in accordance with the present invention will now be described by way of example with reference to the accompanying drawings, in which:

FIGURE 1 is a partly diagrammatic cross-section of the cryostat, and

FIGURE 2 is a cross-section of part of the cryostat on an enlarged scale.

Referring to FIGURE 1, the cryostat is required to keep a body 1 at a low temperature for long periods without frequent refilling of the cryostat with refrigerant being necessary. The refrigerant used in this particular example is liquid argon, although it will be appreciated that other refrigerants having a boiling point below the ambient temperature could be used.

The cryostat comprises a double-walled stainless steel vessel 2 which at the lower end opens out to form a reservoir 3. Closely fitting within the reservoir is a dome 4 the sides of which reach almost to the bottom of the reservoir 3. A tube 5 extends from a point just below the top of the dome 4 up to the top of the cryostat where there is a vent valve 6, and also a control mechanism 7 mounted on a foam plastic plug 8 which partly closes the top of the cryostat.

To fill the cryostat liquid argon is poured through the opening in the plug 8 and the valve 6 is opened manually to vent gas trapped below the dome 4. When the reservoir 3 is full the valve 6 is closed manually. If the liquid argon were all at a uniform temperature, any bubbles evolved inside the reservoir 3 by heat passing along the tube 5 or radiation to the bottom surface of the reservoir 3 would condense again due to the pressure of the liquid argon above the top of the reservoir 3. In practice, however, temperature gradients will occur and argon gas will collect under the dome 4, displacing some liquid argon from the reservoir 3 and raising the level at the top of the cryostat. Thus the liquid argon level will tend to rise rather than fall due to evaporation.

To maintain the liquid argon level constant at a desired distance from the top of the cryostat the control mechanism 7, which is shown in more detail in FIGURE 2 of the drawings, is used. The mechanism 7 comprises a foam plastic float 9 which is coupled by way of a rod 10 to a lever 11. The lever 11 is hinged about an axis 12 and carries a plug 13 which normally closes the end of the tube 5. The rod 10 passes through the lever 11 and has collars 14 and 15 varranged one on each side of the lever 11 at a spacing slightly greater than the width of the lever 11.

When the liquid argon level rises, the float 9 lifts the lever 11 by way of the rod 10 and collar 14, and the end of the tube 5 is uncovered. This allows trapped argon gas to escape from beneath the dome 4 and the liquid argon level falls until the end of the tube 5 is covered again. Condensation on the moving parts of the mechanism 7 is prevented by the intermittent flushing of its enclosure 16 by argon gas escaping from the tube 5.

In a particular embodiment the volume of the reservoir 3 is 12 litres. The evaporation rate is approximately 0.45 litre per hour, so the cryostat needs refilling about every 27 hours.

In this embodiment the body 1 is a block of beryllium which is to be subjected to a beam of neutrons, but clearly the cryostat is not limited to this application.

In the review of the embodiment described by way of illustration the large part of the liquid under the dome 4 causes gas to collect under the dome 4. This displaces liquid from the dome 4 tending to make the upper liquid level at the top of the cryostat rise (rather than fall, which is the usual effect of evaporation of liquid from a container). It is also noted that a tube 5, which is a heat conducting tube as described, conducts heat in a small heat path from the ambient to refrigerant in the inside or second vessel formed by the sides of dome 4 that reach downward to near the bottom of reservoir 3, also as described above, i.e. to the bottom of the cryostat. This small heat path produces a small temperature difi'erence or gradient in the refrigerant in the inner open end vessel 4, as described, whereby gas is slowly evolved in the inner vessel under dome 4 to cause refrigerant to move correspondingly out of the inner vessel into the outer vessel. This causes the liquid level at the top of the outer vessel to rise so as to raise the float thereon to open the valve plug 13. This intermittently vents the gas from under dome 4 and circulates this vented gas around the mechanism 7 in enclosure 16, e.g. by moving the gas down along rod 10 and out the hole in plug 8 to the ambient, thus to prevent condensation on the venting mechanism 7. This venting thereupon causes liquid to How from the outer to the inner vessel thus sequentially to lower the level of the liquid at the top of the cryostat and to close the valve plug 13. Thereby this cycle of venting and valve plug closing is repeated sequentially automatically.

I claim:

1. In an inverted liquid cryostat for a liquid having a boiling point below ambient temperature, comprising a vessel having a reservoir adapted to contain said liquid, the improvement comprising a reservoir having a dome beneath which gas evolved by evaporation collects to raise said liquid level and vent means for said gas from beneath said dome which vents said gas to restore said liquid level to a predetermined liquid level in response to the raising of said liquid level by said gas evolution and collection beneath said dome.

2. A cryostat, comprising a vessel adapted to contain a refrigerant having a boiling point below the ambient temperature of said vessel, the lower part of the vessel forming a reservoir and the upper part of the vessel forming a region for receiving a body to be maintained below the ambient temperature, said reservoir having a hemispherical roof surface dome within the reservoir beneath which gas evolves by evaporation of the refrigerant in the reservoir, said gas evolution displacing some refrigerant from the reservoir to raise the level of the refrigerant at the top of the cryostat, vent means communicating from said ambient to under said dome for venting said gas from beneath said dome to reduce the level of said displaced refrigerant for restoring the refrigerant to a predetermined level, and float means on said liquid for opening and closing said vent means in response to the raising and lowering of said refrigerant from said predetermined level respectively, whereby said predetermined level is maintained for long periods of time.

3. A cryostat comprising a vessel adapted to contain a liquid refrigerant having a boiling point below the ambient temperature of said vessel and a first upper liquid level in said vessel, means forming a hemispherical roof surface dome in said vessel under which said refrigerant has a second liquid level from which gas evolves to raise the first upper liquid level of said refrigerant in said cryostat, means communicating from said ambient to under said dome for venting said gas from beneath said dome to reduce the first upper liquid level of said refrigerant in said cryostat, and means controlling the venting of said gas in response to the raising of said first upper liquid level for restoring said refrigerant to a predetermined first upper liquid level in said cryostat.

4. A cryostat, comprising a vessel adapted to contain a liquid refrigerant having a boiling point below the ambient temperature of said vessel and a first upper liquid level in said vessel, means forming an open ended chamher having sides extending upwardly from adjacent the bottom of said cryostat to form a hemispherical roof surface dome under which said refrigerant has a second lower liquid level on top of which said refrigerant changes to gas that collects under said dome to displace said refrigerant from said container to raise the first upper liquid level of said refrigerant, means communicating from said ambient to under said dome for venting said gas from beneath said dome so that said refrigerant is received into said container to lower said first upper liquid level, and means for controlling said venting to maintain a given first upper liquid level.

5. A cryostat, comprising a first open ended vessel adapted to receive a refrigerant having a boiling point below the ambient temperature of said vessel and a first upper liquid level in said first vessel, an inverted second open ended vessel in said first vessel and communicating therewith having a hemispherical roof surface dome under which said refrigerant changes to gas to displace said refrigerant from said second vessel to raise said first upper liquid level, means for venting said gas from beneath said dome to said ambient for displacing said refrigerant from said first to said second vessel to lower said first upper liquid level, said means for venting said gas from beneath said dome having a heat conducting tube passing from said ambient through the closed end of said first vessel and the open end of said second vessel to a place adjacent the dome of said second vessel to produce a small heat path from said ambient to the refrigerant in said second vessel to produce said gas under said dome from said refrigerant.

References Cited by the Examiner UNITED STATES PATENTS 1,977,267 10/1934 Endacott 6250 2,237,052 4/1941 Gill 62-50 2,293,263 8/ 1942 Kornemann et al 6250 2,856,160 10/1958 Kaye et al. -405 2,861,715 11/1958 Wismiller et a1. 165-105 X 3,045,437 7/1962 Aronson 6245 X 3,046,751 7/ 1962 Gardner 6245 X 3,158,459 11/ 1964 Guilhem 6245 LLOYD L. KING, Primary Examiner.

ROBERT A. OLEARY, Assistant Examiner. 

3. A CRYOSTAT COMPRISING A VESSEL ADAPTED TO CONTAIN A LIQUID REFRIGERANT HAVING A BOILING POINT BELOW THE AMBIENT TEMPERATURE OF SAID VESSEL AND A FIRST UPPER LIQUID LEVEL IN SAID VESSEL, MEANS FORMING A HEMISPHERICAL ROOF SURFACE DOME IN SAID VESSEL UNDER WHICH SAID REFRIGERANT HAS A SECOND LIQUID LEVEL FROM WHICH GAS EVOLVES TO RAISE THE FIRST UPPER LIQUID LEVEL OF SAID REFRIGERANT IN SAID CRYOSTAT, MEANS COMMUNICATING FROM SAID AMBIENT TO UNDER SAID DOME FOR VENTING SAID GAS FROM BENEATH SAID DOME TO REDUCE THE FIRST UPPER LIQUID LEVEL OF SAID REFRIGERANT IN SAID CRYOSTAT, AND MEANS CONTROLLING THE VENTING OF SAID GAS IN RESPONSE TO THE RAISING OF SAID FIRST UPPER LIQUID LEVEL FOR RESTORING SAID REFRIGERANT TO A PREDETERMINED FIRST UPPER LIQUID LEVEL IN SAID CRYOSTAT. 